accel/tcg: Add aarch64 lse2 load_atom_extract_al16_or_al8

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

host/include/aarch64/host/load-extract-al16-al8.h

@@ -0,0 +1,40 @@
+/*
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ * Atomic extract 64 from 128-bit, AArch64 version.
+ *
+ * Copyright (C) 2023 Linaro, Ltd.
+ */
+
+#ifndef AARCH64_LOAD_EXTRACT_AL16_AL8_H
+#define AARCH64_LOAD_EXTRACT_AL16_AL8_H
+
+#include "host/cpuinfo.h"
+#include "tcg/debug-assert.h"
+
+/**
+ * load_atom_extract_al16_or_al8:
+ * @pv: host address
+ * @s: object size in bytes, @s <= 8.
+ *
+ * Load @s bytes from @pv, when pv % s != 0.  If [p, p+s-1] does not
+ * cross an 16-byte boundary then the access must be 16-byte atomic,
+ * otherwise the access must be 8-byte atomic.
+ */
+static inline uint64_t load_atom_extract_al16_or_al8(void *pv, int s)
+{
+    uintptr_t pi = (uintptr_t)pv;
+    __int128_t *ptr_align = (__int128_t *)(pi & ~7);
+    int shr = (pi & 7) * 8;
+    uint64_t l, h;
+
+    /*
+     * With FEAT_LSE2, LDP is single-copy atomic if 16-byte aligned
+     * and single-copy atomic on the parts if 8-byte aligned.
+     * All we need do is align the pointer mod 8.
+     */
+    tcg_debug_assert(HAVE_ATOMIC128_RO);
+    asm("ldp %0, %1, %2" : "=r"(l), "=r"(h) : "m"(*ptr_align));
+    return (l >> shr) | (h << (-shr & 63));
+}
+
+#endif /* AARCH64_LOAD_EXTRACT_AL16_AL8_H */